This invention relates generally to gas turbine engines and more particularly, to combustors for use with gas turbine engines.
At least some known gas turbine engines ignite and combust a fuel and air mixture to release heat energy from the fuel in an exothermic reaction to form a high temperature combustion gas stream. At least one by-product of the combustion reaction may be subject to regulatory limitations. For example, within thermally-driven reactions, nitrogen oxide (NOx) may be formed by a reaction between nitrogen and oxygen in the air initiated by the high temperatures within the gas turbine engine. Generally, engine efficiency increases as the combustion gas stream temperature entering a turbine section of the engine increases. However, increasing the combustion gas temperature may facilitate an increased formation of NOx.
Combustion normally occurs at or near an upstream region of a combustor that is normally referred to as the reaction zone or the primary zone. Additional mixing and combusting of fuel and air may occur downstream of the reaction zone in a region often referred to as a dilution zone. Air may be introduced directly into the dilution zone to dilute the fuel and air mixture to facilitate achieving a predetermined temperature of the gas stream entering the turbine section.
To facilitate controlling (including decreasing) NOx emissions during turbine engine operation, at least some known gas turbine engines use annular combustors that operate with a lean fuel/air ratio, i.e., the combustors are operated in a manner wherein fuel is premixed with air prior to admission into the combustor's reaction zone. The operable range of the engine using premixed fuel and air at certain fuel/air ratios may be limited. This is due to a particular premixed fuel/air ratio within a certain range of combustion operations may not facilitate a similar flame stability within other ranges. For example, at least some known engines operate primarily with premix flame conditions over a predetermined portion of the engine's operating range. While operating outside of this range, diffusion combustion, wherein the air and fuel are mixed within the associated combustion region, is often used to supplement and/or replace premix combustion. Diffusion combustion may have a tendency to increase the potential for NOx formation. Therefore, at least some of these known engines that operate with a lean premixed combustion reaction divert a portion of the combustion supply air to the dilution region downstream from the reaction zone and into the gas stream for improved ignition and operation of the engine within operating ranges other than those that exhibit a predetermined flame stability using premixed combustion exclusively. This method is sometimes referred to as “bypassing” since some air bypasses the combustion reaction. Within such engines the fuel/air ratio is generally not constant over the rated range of combustion operation of the engine. For example, a fuel/air ratio that facilitates reducing combustion reaction temperatures during normal engine operations may not be sufficient during start-up or lower power operations of the gas turbine engine.